Can Republicans defy midterm history and strengthen their position in the Senate? The answer might hinge on a state the party has until recently taken for granted — and which thus far looks surprisingly resistant to Republican messaging.

The battle for the Senate could come down to Arizona, where two sitting congresswomen — Democrat Kyrsten Sinema and Republican Martha McSally — are locked in a very tight race to fill the seat being vacated by retiring GOP Sen. Jeff Flake. The RealClearPolitics poll average puts Sinema 0.2 percent ahead. This lead is in spite of a constant stream of embarrassing revelations from Sinema — which could signal difficulties for the GOP on Tuesday.

The electoral picture still looks difficult for Republicans in the House, even after the surge of enthusiasm among President Trump's voters after Brett Kavanaugh's confirmation. The generic congressional ballot gap has remained mainly steady throughout October, with the RCP average starting the month at Democrats +7.4 and ending it at Democrats +7.6. More targeted polling puts the two parties in a virtual dead heat among competitive districts, but almost three dozen retirements by Republican incumbents make the prospect of keeping losses under the 23 seats Democrats need for a majority a tough project, at the very least.

The news looks much better for Republicans in Senate elections, where the numbers and vulnerabilities reverse themselves. Democrats have to defend 26 seats (including special elections), with 10 in states Trump won two years ago. The GOP only has to defend nine seats, and only one in a state where Hillary Clinton won in 2016 — Nevada.

Even in Nevada, however, the incumbent Republican appears to have begun pulling out in front of a tough challenge from Democratic Rep. Jacky Rosen, despite heavy spending by Democrats in the state. In other red states, such as North Dakota, Missouri, and Indiana, Republicans appear to have strong chances to pick up seats, and in Florida and perhaps Montana and Minnesota too.

But … there's still Arizona, and a nagging problem for the GOP.

Sinema spent most of the cycle with a substantial polling lead. The retiring Flake had crossed swords often with Trump, perhaps reflecting an electorate that favored the late John McCain's center-right politics over Trump's populist appeal; Trump only won Arizona 49 percent to 46 percent two years ago. Since mid-September, the Republican McSally's polling support has grown, making this into a toss-up, and most expected the fallout of the Kavanaugh hearings to boost her fortunes further as Republican voter enthusiasm swelled.

Instead, though, polling numbers for both candidates declined over the month. Sinema has led in the two most recent polls in RCP's aggregation, but only near the margins of error. Even while Republicans fire off their oppo-research bombs at Sinema from her years as a far-left political activist (including video clips where Sinema denigrates Arizona voters), McSally does not seem to be making up ground. Even the withdrawal of an endorsement from the Arizona State Troopers Association didn't seem to damage Sinema; an NBC/Marist poll five days later put her up six points.

Oddly, though, the GOP doesn't have the same problem in its other high-profile statewide race. Incumbent GOP Gov. Doug Ducey has maintained double-digit leads over his Democratic challenger David Garcia in every poll over the past month. Ducey has captured over 50 percent of the respondents in the last six polls in RCP's aggregation, all the way back to a mid-September poll from NBC/Marist. Whatever is holding McSally back isn't hampering Ducey, and whatever is keeping Sinema competitive isn't helping Garcia.

Furthermore, Republicans didn't have any enthusiasm trouble even before the Kavanaugh hearings. Ducey's relatively uncompetitive primary at the end of August drew 29 percent more overall votes than the more closely contested three-way Democratic race than produced Garcia's nomination. For that matter, 28 percent more Republicans cast ballots in their Senate primary than Democrats did in theirs. That large enthusiasm gap would normally suggest that Republicans would dominate in the general election, especially with a primary just a little over two months before it.

So what's the problem with Republicans since the primary? McSally's primary win came after a spectacularly bitter fight with two well-known Trump-supporting populists, former Sheriff Joe Arpaio and Dr. Kelly Ward. McSally didn't just win on a vote-splitting fluke, though; she won a solid majority of the overall vote, almost double that of runner-up Ward at 55 percent to 28 percent. The primary fight reflected bitter divisions within the GOP that Trump and Flake exacerbated over the past two years, both inside and outside Arizona, and which at times involved the late McCain as well.

Polling numbers suggest that the party has not yet fully unified behind McSally after that messy primary, which largely left Ducey unscathed. Per a CBS/YouGov poll last week, 9 percent of Republican voters plan to back Sinema, while only 1 percent of Democrats support McSally. That wiped out a three-point edge for McSally among independents. Unless McSally can find a way to bring home those disaffected Republicans, Republicans may lose a seat they should have been able to hold.

That leaves Republicans in a precarious position in Arizona even beyond this election. Jon Kyl, appointed by Ducey to fill McCain's seat after his death, has indicated that he will resign at the end of the year, allowing Ducey to appoint another Republican for the remaining two years of McCain's last term. Ducey could appoint McSally if she loses, but she's already proven herself vulnerable. Regardless, however, Democrats will now know that Arizona is ripe for a takeover in 2020 as well.